Electronic devices such as mobile phone, PDA (Personal Digital Assistant), pager, etc. usually have a display screen, which is mostly a liquid crystal displaying apparatus that could make the content to be displayed visible by providing backlights when the luminous intensity of the circumstance is not high enough. Meanwhile, these electronic devices such as mobile phone and PDA (Personal Digital Assistant) may also have a data inputting apparatus comprising a set of keys, and the devices make the keypad visible through backlights when the luminous intensity of the circumstance is not high enough.
FIG. 1 is the prior art digital luminance controlling system. The system is a technical solution disclosed in the U.S. Pat. No. 5,760,760 (granted on Jun. 2, 1998), comprising a light sensing apparatus 110, a digital luminance controlling apparatus 120, a light source controlling apparatus 130 and a group of light sources 140, with the digital luminance controlling apparatus further comprising an analog-to-digital converter (ADC) 122, a digital signal processor (DSP) 124 and a memory 126. When the light sensing apparatus 110 detects the luminous intensity of the circumstance, it sends an signal of the luminous intensity of the circumstance to the digital luminance controlling apparatus 120, and the analog signal is converted into digital signal in accordance with a preset sampling frequency by the analog-to-digital converter 122 and the digital signal is sent to the digital signal processor 124, and the digital signal processor 124 reads the luminance level in the memory 126 according to the digital luminous intensity signal and converts it into a luminance controlling signal to be sent to the light control device 130 which adjusts the luminance of the light source according to the received luminance controlling signal.
FIG. 2 is the light source controlling apparatus of the prior art digital luminance controlling system. The device is a technical solution disclosed in the British Patent GB2, 365, 691 (published on Feb. 20, 2002), comprising a group of selection switches (S1, S2, . . . , SN) and M groups of resistors (R11, R12, . . . , R1N; R21, R22, . . . , R2N; . . . ; RM1, RM2, . . . , RMN), with the number of resistors in each group of resistors depending on the number of selection switches, i.e., on the number of luminance levels. The group of selection switches and M groups of resistors are connected to a light source, i.e., an illuminating apparatus comprising a group of luminaries (L1, L2, . . . , LM) (such as light-emitting diodes), in such a manner that each switch, such as S1, is connected to a group of resistors (R11, R21, . . . , RM1) and the luminaries (L1, L2, . . . , LM), wherein the number of resistors in the group of resistors depends on the number of the luminaries, and wherein the resistors are directly connected to the luminaries. By setting different switches to the connection state, the currents through the luminaries or the voltages applied to the luminaries are made different, thereby, the luminance of the luminaries is adjusted.
Since the number of luminance levels in the prior art is limited by the number of the selection switches, with the increasing of luminance levels, the number of selection switches and the number of the groups of the resistors increase simultaneously, thus the selection of the number of luminance levels is obviously limited while the corresponding cost increases. Therefore, the object of reducing power consumption by changing the luminance more smoothly with the change of the luminous intensity of the circumstance cannot be achieved.
Hence, an improved luminance controlling system is needed, which could change the luminance more smoothly with the change of the luminous intensity of the circumstance so as to achieve the objects of reducing power consumption and saving cost.